Presently, many portable devices such as cordless power tools, computer notebooks and mobile phones are shipped with a lithium-ion battery pack due to its advantages of high energy density, low self-discharge, no memory effect, longer run-time and light-weight compared to a conventional battery pack. However, lithium-ion battery packs may result in unsafe operation due to over-charging, over-discharging or over-heating conditions.
A lithium-ion battery pack is typically made up of one or a plurality of lithium-ion cells either in series or in parallel connection, depending on its output energy requirements. The battery pack also comprises a protection board for monitoring and ensuring that the lithium-ion cells operate within their safety limits. Some battery packs include fuel gauge integrated circuit (IC) to estimate the state of charge (SOC) and are also able to communicate with external devices via System Management Bus (SMBus) communication. This type of battery pack is commonly known as a Smart Battery Pack.
As part of the manufacturing process, battery pack manufacturers typically perform a battery pack burn-in test using a 3-phase cycle of full charging-full discharging-50% charging of each battery pack to weed out any initial faulty battery packs due to component defects, mismatched cells, poor spot-welding, poor solder joint and other functional defects. At the same time, the battery pack also performs a SOC calibration to accurately update its SOC during the full charging-full discharging cycle.
The battery pack burn-in test system is a relatively sophisticated electronic test system which supports multi-channel testing, user programmable burn-in test parameters and test data logging functions. The devices which provide the transfer of energy to and from the battery packs are the Constant Current Constant Voltage (CCCV) Charger and the Electronic Load (ELoad) respectively. During the discharge phase, the battery pack to be tested, here referred to as the Pack-Under-Test (PUT), is connected to an ELoad which discharges the PUT with a preset current. The PUT will terminate the discharging process when any of its cells goes below the over-discharge voltage threshold. In the charge phase, the PUT is connected to a CCCV Charger where the charger will terminate the charging process when the PUT's full-charge conditions are met. The burn-in test cycle typically begins with charging and the PUT will first be charged from an initial capacity of about 50% to a full-charge capacity of 100%. The PUT will then be completely discharged to 0%. Finally, the PUT is recharged to its shipping capacity of 50%. The initial capacity of the PUT is about 50% as this is the initial shipping capacity of the lithium-ion cells. Accordingly, the battery pack manufacturers have to recharge the PUT to 50% prior to shipment. The duration of the burn-in test cycle depends on the setting of the charging and discharging currents. More heat will be generated for a shorter burn-in period due to higher current requirements and vice versa. Typically, the burn-in test duration ranges from 3 hours to 5 hours.
The present method of battery pack burn-in test process has a problem of generating large amount of heat as the PUT exchanges energy between the ELoad and the CCCV charger. As an example, a standard 6-cell lithium-ion battery pack that is designed for a computer notebook typically dissipates around 35 W in the form of heat during the discharge phase. Consequently, a thousand PUTs discharging at the same time will result in 35 kW of power being converted into waste heat. It is relatively common for a battery pack manufacturing site to maintain burn-in processes of thousands of battery packs simultaneously. With such a large amount of heat generated as a result of the burn-in processes, it is a very costly operation to control the temperature of the burn-in process room to an acceptable operating temperature. Powerful air-conditioners and heat removal system may help to cool down the room but these consume additional electricity which can result in higher costs. Furthermore, failure to control the temperature of the burn-in process room may result in safety concerns as the lithium-ion battery packs may be operating outside their safety operating zone.
A need therefore exists to provide a battery pack burn-in test system and method that seek to address at least one of the abovementioned problems.